The field is internal combustion engines. Particularly, the field relates to two-stroke engines with ported cylinders. In more particular applications, the field relates to constructions and methods for releasing charge air from a ported cylinder equipped with opposed pistons so as to enable engine braking.
When compared with four-stroke engines, ported, two-stroke, opposed-piston engines have acknowledged advantages of specific output, power density, and power-to-weight ratio. For these and other reasons, after almost a century of limited use, increasing attention is being given to the utilization of opposed-piston engines in a wide variety of modern transportation applications. A representative opposed-piston engine is illustrated in FIGS. 1 and 2. As seen in FIG. 1, the opposed-piston engine includes one or more cylinders 10, each with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Each of one or more fuel injector nozzles 17 is located in a respective injector port that opens through the side of the cylinder, at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred as the “exhaust” piston because of its proximity to the exhaust port 14; and, the end of the cylinder wherein the exhaust port is formed is referred to as the “exhaust end”. Similarly, the piston 22 is referred as the “intake” piston because of its proximity to the intake port 16, and the corresponding end of the cylinder is the “intake end”.
Opposed Piston Fundamentals:
Operation of an opposed-piston engine with one or more cylinders 10 is well understood. In this regard, and with reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) locations where they are at their innermost positions in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) locations in which they are at their outermost positions in the cylinder. In a useful, but not a necessary aspect of opposed-piston engine construction, a phase offset is introduced in the piston movements that produces a sequence in which the exhaust port 14 opens as the exhaust piston 20 approaches BDC before the intake port 16 opens so that exhaust gasses produced by combustion start to flow out of the exhaust port 14. As the intake piston approaches BDC, the intake port 16 opens and a charge of pressurized air (“charge air”), with or without recirculated exhaust gas, is forced into the cylinder 10. The charge air entering the cylinder drives exhaust gasses out of the exhaust port 14; this process is referred to as “scavenging”.
As per FIG. 1, presuming the phase offset mentioned above, after the exhaust port 14 is closed, the intake port 16 closes and the charge air remaining in the cylinder is compressed between the end surfaces 20e and 22e. Typically, the charge air is swirled as it passes through the intake port 16 to promote scavenging while the ports are open and, after the ports close, to mix the air with the injected fuel. The fuel is typically diesel, which is injected into the cylinder by one or more high pressure injectors. With reference to FIG. 1 as an example, the swirling charge air 30 has a generally helical motion that forms a vorticity in the bore which circulates around the longitudinal axis of the cylinder. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 is injected through a nozzle 17 directly into the swirling charge air 30, between the end surfaces 20e, 22e of the pistons. The swirling mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 move through their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations. In two-stroke engines, the process of compressing air to obtain ignition of fuel injected into the air is referred to as “compression ignition”.
Release of compressed charge air is advantageous in some aspects of diesel engine operation. Engine braking that is synchronized with the stroke-cycle of the engine (also called “decompression braking” and “compression-release braking”) is particularly useful feature for medium and heavy duty trucks equipped with diesel engines. Compression-release braking is activated in a valved, four-stroke diesel engine by halting fuel injection, closing EGR valves, and releasing compressed charge air from the cylinder when the piston is at or near the top of its compression stroke, immediately before the expansion stroke begins. Releasing the compressed air at this point releases energy that would otherwise urge the piston from top to bottom dead center during the expansion stroke. This significantly reduces the work extracted from the pistons as they return to BDC, which produces the desirable braking effect.
In valved engines constructed for compression-release braking, the compressed air is released by opening an exhaust valve out of sequence at or near the end of the compression stroke. The compressed air flows through the open valve into the exhaust system. At BDC, charge air is again admitted to the cylinder. As the cycle repeats, potential engine energy is discarded by release of the compressed air, which causes the engine to slow down. Compression-release braking significantly enhances the braking capability of medium and heavy duty vehicles, thereby making them safer to operate, even at higher average speeds. Furthermore, in contributing significant additional braking capacity, a compression-release braking system extends the lifetime of the mechanical braking systems in medium and heavy duty trucks, which reduces the costs of maintenance over the lifetime of such vehicles.
Compression-release braking constructions for four-stroke engines typically operate in response to a manually-generated signal accompanied by release of the throttle. When engine braking is activated, the cylinder is vented through an exhaust valve that is opened out of sequence during the compression stroke. In a representative embodiment of compression-release braking in a four-stroke engine, U.S. Pat. No. 4,473,047 teaches the provision of two exhaust valves per cylinder. During normal operation, both valves are open during the exhaust stroke. When compression-release braking is actuated, one of the exhaust valves is opened at or near TDC of the compression stroke.
In a simpler and more easily-implemented mode of engine braking, exhaust and/or intake valves are held open continuously throughout a braking period while the supply of fuel is interrupted. Substantial braking power is thereby generated, without the need for synchronization to the engine operating cycle. U.S. Pat. No. 3,547,087 teaches the use of a hydraulically-actuated blocking arm that advances to block return movement of a rocker arm associated with a valve. With the return oscillation of the rocker arm blocked, the associated valve is kept at least partially opened until the blocking arm is retracted. Charge air provided to the cylinder is continuously returned through the open valve to the exhaust channel.
Conventional four-stroke diesel engines achieve the advantages of engine braking by modifications of intake and/or exhaust valve mechanisms that are designed to interrupt the normal operation of engine valves during certain portions of the engine operating cycle or throughout the entire cycle. The intake and exhaust valves are supported in a cylinder head and have associated actuation machinery. However, two-stroke opposed-piston engines do not include valves or cylinder heads. Instead, they intake charge air and exhaust combustion products through cylinder ports that are separated longitudinally on the cylinder and controlled by the pistons. Accordingly, without a cylinder head and intake and exhaust valves, an opposed-piston engine cannot incorporate the engine braking solutions tailored for valved diesel engines. Nevertheless, the addition of engine braking to opposed-piston engine operation would confer benefits and advantages that are realized by valved engines with these capabilities. Accordingly, there is a need for two-stroke, opposed-piston cylinder constructions that provide engine braking.